Baseplates or bases for data recording disk drives, such as rigid magnetic recording disk drives, are complex three-dimensional structures that provide a rigid and precise platform upon which multiple mechanical and electronic components are mounted. Prior art disk drives typically use a machined die-cast aluminum baseplate since it has the following properties: 1) a die-cast structure allows a variety of three-dimensional geometries; 2) metal material has high strength and dimensional stability; 3) a die-cast structure can be easily machined to provide tight-tolerance datums and features to accept fasteners; 4) the material is electrically conductive for electrical shielding; and 5) the material is low cost.
Die-cast aluminum baseplates have the following limitations: 1) it is difficult to achieve tolerances tighter than about 0.15 mm without post-machining; 2) wall thicknesses less than 0.5 mm are difficult to achieve; 3) tight-tolerance features are limited to shapes which can be made by casting and post-machining with an end-mill; and 4) they typically require a polymer coating to guarantee an air-tight seal for contamination control and to prevent surface corrosion. While these limitations have not posed problems for large, e.g., 2.5" or 3.5" form factor disk drives, they place significant constraints on baseplates designed for smaller drives, such as a miniature disk drive designed for digital cameras and hand-held or palm-size computing devices. In addition, the cost advantage of die-cast aluminum disappears for small baseplates since the bulk material cost becomes small compared to the machining and coating costs, which do not scale down linearly with baseplate size.
Plastic has been considered previously as a baseplate material for large disk drives. U.S. Pat. No. 5,025,335 describes a 3.5" disk drive with a base and cover of molded plastic. However, the use of plastic alone for a baseplate has several disadvantages: 1) plastic requires a conductive coating or filler in order to be electrically conducting, and even with such measures, its conductivity is inferior to aluminum; 2) low-cost plastics are less rigid than aluminum; 3) dimensional stability is inferior to aluminum due to creep and lower phase transition temperature; and 4) some plastic materials can outgas harmful vapors into the interior of the disk drive.
A composite sheet metal and plastic base for a 2.5" removable disk drive that uses a plastic skeleton-like sheet with a sheet metal insert in an inner portion of the plastic sheet is described in U.S. Pat. No. 5,243,495. The metal insert, which supports the disk drive actuator and spindle motor, is glued or heat staked to the plastic sheet at various locations around its periphery. This semi-flexible bonding of the metal insert to the plastic sheet is designed to provide shock damping for the components mounted on the metal insert. Because the metal insert covers only an inner portion of the plastic sheet the composite base does not provide rigidity to the base and does not electrically shield the components from the electronics card mounted beneath the base.
What is needed is a disk drive base that is sufficiently rigid and provides adequate electrical shielding, but is also low cost and allows tolerances superior to die casting, such that post-machining of datums is not necessary.